Gas gauge compatible with vacuum environments

ABSTRACT

In one embodiment of the present invention, there is provided a gas gauge for use in a vacuum environment having a measurement gas flow channel. The gas gauge may comprise a measurement nozzle in the measurement gas flow channel. The measurement nozzle may be configured to operate at a sonically choked flow condition of a volumetric flow being sourced from a gas supply coupled to the measurement gas flow channel. The gas gauge may further comprise a pressure sensor operatively coupled to the measurement gas flow channel downstream from the sonically choked flow condition of the volumetric flow to measure a differential pressure of the volumetric flow for providing an indication of a gap between a distal end of the measurement nozzle and a target surface proximal thereto.

This patent application is related to U.S. application Ser. No.12/809,171, which is incorporated by reference herein in its entirety.

BACKGROUND

1. Field of the Invention

The present invention relates to the Acid of gas gauges suitable for usein vacuum environments of a lithographic apparatus.

2. Related Art

Lithography is a process used to create features on the surface ofsubstrates. Such substrates can include those used in the manufacture offlat panel displays, circuit boards, various integrated circuits, andthe like. A frequently used substrate for such applications is asemiconductor wafer. One skilled in the relevant art would recognizethat the description herein would also apply to other types ofsubstrates.

During lithography, a wafer, which is disposed on a wafer stage (WS), isexposed to an image projected onto the surface of the wafer by anexposure system located within a lithography system. The exposure systemincludes a reticle (also called a mask) for projecting the image ontothe wafer.

The reticle is usually mounted on a reticle stage (RS) and generallylocated between the wafer and a light source. In photolithography, thereticle is used as a photo mask for printing a circuit on the wafer, forexample. Lithography light shines through the mask and then through aseries of optical lenses that shrink the image. This small image is thenprojected onto the wafer. The process is similar to how a camera bendslight to form an image on film. The light plays an integral role in thelithographic process. For example, in the manufacture of microprocessors(also known as computer chips), the key to creating more powerfulmicroprocessors is the size of the light's wavelength. The shorter thewavelength, the more transistors can be formed on the wafer. A waferwith many transistors results in a more powerful, faster microprocessor.

As chip manufacturers have been able to use shorter wavelengths oflight, they have encountered a problem of the shorter wavelength lightbecoming absorbed by the glass lenses that are intended to focus thelight. Due to the absorption of the shorter wavelength light, the lightfails to reach the silicon wafer. As a result, no circuit pattern iscreated on the silicon wafer. In an attempt to overcome this problem,chip manufacturers developed a lithography process known as ExtremeUltraviolet Lithography (EUVL). In this process, a glass lenses can bereplaced by a mirror.

Photolithographic exposure tools map wafer topography in order to setfocus. They typically employ an array of sensors lined up next to eachother. Topographic data is taken form each one and stored, then analgorithm is employed to establish the best plane for the exposure step.However, optical means of determining focus positioning are subject toerrors from interfering wave fronts from lower layers.

One alternate to the optical means is an air gauge since an air gaugedoes not suffer from the effects generally associated with optical meansof determining focus positioning. An air gauge as an auxiliary focussensor may be capable of detecting the topography of wafers with higherfidelity than the (optical) level sensor. Significant potential existsto expand the capabilities of the air gauge to the point that it becomesa viable replacement for a leveling sensor. Besides being a moreaccurate meteorology device, particularly in an optically noisyenvironment of processed wafers, it is considerably less expensive andit occupies a significantly smaller volume.

Among developmental challenges to be overcome before such advancementcan take place, there appears to be two prominent challenges. Firstly,since a typical air gauge has a relatively long response time, it limitsthe useful bandwidth to approximately ˜50 Hz. Secondly, the fluidicresponse of the air gauge moving over the wafer topography with finitevelocity may need adequate optimization of various controllingparameters. For example, shortening the response time of the air gaugerequires a faster mass flow sensor (an internal component of the airgauge) and possibly shrinking the volume of the air passages.

Moreover, next generation lithography machines may use a vacuumenvironment to eliminate absorption losses and contamination. Operatingan air gauge in these conditions will change the pneumatic operatingconditions from low speed viscous flow to high speed, reaching sonicconditions. The high speed will produce much larger gas flow than can beaccommodated in a vacuum environment and simple inlet throttling willreduce the bridge flow rates to levels that cannot be measured.

SUMMARY

What is needed is an air gauge for use in vacuum environments suitablefor a lithographic apparatus.

In one embodiment of the present invention, there is provided a gasgauge for use in a vacuum environment having a measurement gas flowchannel. The gas gauge may comprise a measurement nozzle in themeasurement gas flow channel. The measurement nozzle may be configuredto operate at a sonically choked flow condition of a volumetric flowbeing sourced from a gas supply coupled to the measurement gas flowchannel. The gas gauge may further comprise a pressure sensoroperatively coupled to the measurement gas flow channel downstream fromthe sonically choked flow condition of the volumetric flow to measure adifferential pressure of the volumetric flow for providing an indicationof a gap between a distal end of the measurement nozzle and a targetsurface proximal thereto.

According to another embodiment of the present invention, there isprovided a lithography apparatus comprising a gas supply and a gas gaugefor use in a vacuum environment. The gas gauge having a measurementnozzle which may be configured to operate at a sonically choked flowcondition of a constant volumetric flow being supplied by the gassupply. The gas gauge may include a pressure sensor operatively coupledto the measurement nozzle downstream from the sonically choked flowcondition of the volumetric flow to measure a differential pressure ofthe volumetric flow for providing an indication of a gap between adistal end of the measurement nozzle and a substrate proximal thereto.The lithography apparatus may further comprise an exposure stationconfigured to expose the substrate to a beam of radiation comprising apattern in its cross section.

According to a further embodiment of the present invention, there isprovided a method for determining a focus position to set focus in aphotolithographic exposure tool by mapping topography of a wafer in avacuum environment. The method may comprise providing a gas supply andusing in the vacuum environment a gas gauge having a measurement nozzleconfigured to operate at a sonically choked flow condition of a constantvolumetric flow being supplied by the gas supply. The gas gauge mayinclude a pressure sensor operatively coupled to the measurement nozzledownstream from the sonically choked flow condition of the volumetricflow to measure a differential pressure of the volumetric flow forproviding an indication of a gap between a distal end of the measurementnozzle and a substrate proximal thereto. The method may further compriseexposing the substrate to a beam of radiation comprising a pattern inits cross section.

Further embodiments, features, and advantages of the present inventions,as well as the structure and operation of the various embodiments of thepresent invention, are described in detail below with reference to theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate one or more embodiments of the presentinvention and, together with the description, further serve to explainthe principles of the invention and to enable a person skilled in thepertinent art to make and use the invention.

FIG. 1 schematically illustrates a side view of a gas gauge inaccordance with one embodiment of the present invention;

FIG. 2 schematically illustrates a side view of an air gauge for use ina vacuum environment according to one embodiment of the presentinvention;

FIG. 3 schematically illustrates a plot showing a flow rate of air as afunction of pressure to realize a sonically choked flow condition of avolumetric flow according to one embodiment of the present invention;

FIG. 4 schematically illustrates formation of annulus according to anembodiment of the present invention;

FIG. 5 schematically illustrates a plot showing pressure as a functionof a gap for indicating gain according to one embodiment of the presentinvention;

FIG. 6 schematically shows a block diagram of a method for determining afocus position to set focus in a photolithographic exposure tool bymapping topography of a wafer in a vacuum environment according to oneembodiment of the present invention; and

FIG. 7 depicts a lithographic apparatus such as an EUV photolithographicsystem according to an embodiment of the present invention.

The present invention will now be described with reference to theaccompanying drawings. Additionally, the left-most digit(s) of areference number can identify the drawing in which the reference numberfirst appears.

DETAILED DESCRIPTION

While specific configurations and arrangements are discussed, it shouldbe understood that this is done for illustrative purposes only. A personskilled in the pertinent art will recognize that other configurationsand arrangements can be used without departing from the spirit and scopeof the present invention. It will be apparent to a person skilled in thepertinent art that this invention can also be employed in a variety ofother applications.

This specification discloses one or more embodiments that incorporatethe features of the present invention involving gas gauges suitable foruse in vacuum environments of a lithographic apparatus, for example, inExtreme Ultraviolet Lithography (EUVL) Systems. The disclosedembodiment(s) merely exemplify the invention. The scope of the inventionis not limited to the disclosed embodiment(s). The invention is definedby the claims appended hereto.

The embodiment(s) described, and references ire the specification to“one embodiment”, “an embodiment”, “an example embodiment” etc.,indicate that the embodiment(s) described can include a particularfeature, structure, or characteristic, but every embodiment cannotnecessarily include the particular feature, structure, orcharacteristic. Moreover, such phrases are not necessarily referring tothe same embodiment. Further, when a particular feature, structure, orcharacteristic is described in connection with an embodiment, it isunderstood that it is within the knowledge of one skilled in the art toeffect such feature, structure, or characteristic in connection withother embodiments whether or not explicitly described.

Embodiments of the present invention provide gas gauges suitable for usein vacuum environments of a lithographic apparatus, for example, inExtreme Ultraviolet Lithography (EUVL) Systems. In a gas gauge, such asan air gauge by depressing an inlet pressure while still allowing anozzle to reach sonic conditions, the requirement for low flow may beobtained. One advantage of this is seen in low Reynolds Numbersdeveloped throughout the sensor which minimizes turbulent pneumaticnoise. This flow condition is substantially different than conventionalair gage sensors.

According to one embodiment of the present invention, by allowing thenozzle to operate above the choking point, the system becomesinsensitive to changes in downstream pressure. This feature essentiallyeliminates the need to monitor the operating pressure ratio of thesystem.

Due to a low flow condition, a differential pressure sensor may ideallybe used to measure the pressure response to changes in a sensoroperating height. In order to limit the range of pressures that thesensor will see, a secondary nozzle may be used only to limit thedifferential pressure seen by the sensor and minimize errors introducedby an unsteady input.

For a gas gauge, a nozzle may be employed at a small operating gap,typically (but not limited to) 135 um. The inlet of the nozzle may bethrottled using a porous restrictor set to produce a fixed nozzle inletpressure at a desired flow rate. The nozzle may be allowed to reachsonic conditions at a nozzle exit, which develops a unique relationbetween flow, pressure and annulus. By holding the flow rate constant,the pressure may be solely a function of the annulus area which isindicative of the height that the nozzle is operating at.

Consistent with one embodiment of the present, invention, an air gaugemay be configured to produce many times, such as a 3-4 times morepneumatic gain than a conventional air gauge sensor. According toanother embodiment of the present invention, a sealed housing maycontain all elements that produce contaminants and may be continuouslypurged to limit the impact of even small leakage.

Process errors are inherent with optical topography systems. They arealso strongly dependent upon the type and thickness of the resist usedfor a given process. In an EUV system, a resist used may be thinner thanthat employed in other processes. This means that there is an increasedneed for a process independent topographic system which can operate invacuum. Although it is not intuitive that an air gauge can function withlittle air, or that a nozzle operating at sonic conditions could bequiet enough for accurate gap measurements, an air gauge consistent withone embodiment of the present invention may generally meet suchrequirements. This air gauge may be applied as a calibrator or as aprimary sensor for establishing surface topography.

FIG. 1 schematically illustrates a side view of a gas gauge 100 that maybe compatible with vacuum environments in accordance with one embodimentof the present invention. For example, the gas gauge 100 may be suitablefor use in a vacuum environment having a measurement gas flow channel105. In an alternate embodiment however, the gas gauge 100 may also beused in an ambient environment.

Consistent with one embodiment of the present invention, the gas gauge100 may comprise a measurement nozzle 110 in the measurement gas flowchannel 105. The measurement nozzle 110 may be configured to operate ata sonically choked flow condition in an area 115 of a volumetric flowbeing sourced from a gas supply 120 coupled to the measurement gas flowchannel 105. The gas supply 120 may include an air pump (not shown) thatsupplies air to a flow controller (not shown) for maintaining a constantrate of air flow into the gas gauge 100. The air may pass through afilter (not shown) before entering into the measurement gas flow channel105, which ultimately leads to the measurement nozzle 110. To ensure acommon flow rate to the measurement nozzle 110 and a secondary pressureport 122, a first flow restrictor 125 may be placed in the measurementgas flow channel 105. The first flow restrictor 125 may create thesonically choked flow condition and have the effect of damping outupstream pressure and flow oscillations or disturbances.

The gas gauge 100 may further comprise a pressure sensor 127 operativelycoupled to the measurement gas flow channel 105 downstream from thesonically choked flow condition area 115 of the volumetric flow tomeasure a differential pressure of the volumetric flow for providing anindication of a gap 130 between a distal end 135 of the measurementnozzle 110 and a target surface 140 of a target 145 proximal thereto.

In one embodiment of the present invention, the pressure sensor 127 maybe a differential pressure sensor. One example of the differentialpressure sensor is a capacitive pressure sensor having a bandwidth of1000 Hz and a size less than 25 mm cube. Persons ordinarily skilled inthe pertinent art would recognize that such type of differentialpressure sensors are widely used and are generally available from MKSInstruments of Andover, Mass. Consistent with one embodiment of thepresent invention, the pressure sensor 127 may comprise a pressurereceiving diaphragm having a conductive layer, and a fixed electrodelocated opposite to the diaphragm and connected to a capacitancedetecting circuit. The capacitance detecting circuit mayelectrostatically sense displacement of the inner portion of thediaphragm to detect a differential pressure, by using fluctuation incapacitance produced between the diaphragm and the fixed electrode.

The measurement gas flow channel 105 may be connected via a differentialflow channel 105 a. The differential flow channel 105 a may include thepressure sensor 127 between the measurement gas flow channel 105 and thesecondary pressure port 122. If the pressure at the measurement nozzle110 is substantially equal to the pressure at the secondary pressureport 122, there is no flow across the pressure sensor 127. However, ifthe distance between the measurement nozzle 110 and, for example, thetarget 145 such as a wafer changes relative to the distance between thedistal end 135 of the measurement nozzle 110, the pressure at themeasurement nozzle 110 will also change. A movement of air to or from,for example, the secondary pressure port 122 and the measurement gasflow channel 105 creates a differential pressure across the differentialflow channel 105 a. This difference in pressure is detected by thepressure sensor 127. For example, the pressure sensor 127 may measurethe differential pressure of the volumetric flow between a firstpressure level of the measurement nozzle 110 and a second pressure levelof the secondary pressure port 122. Once detected, the distance betweenthe measurement nozzle 110 and the target surface 140 may be measured.

In this way, the gas gauge 100 may provide a measure of an offsetdistance in the gap 130 based on a signal of the differential pressurefrom the pressure sensor 127. For determining a focus position bymapping topography of a wafer, the first flow restrictor 125 may belocated upstream of the pressure sensor 127 and configured to depresspressure at an inlet 150 of the measurement gas flow channel 105 whileallowing the measurement nozzle 110 to reach the sonically choked flowcondition for gas flowing at a substantially constant flow rate from thegas supply 120. The inlet 150 of the measurement gas flow channel 105may be configured to receive the gas from the gas supply 120.

In one embodiment, the gas supply 120 and the first flow restrictor 125may be configured to maintain a ratio between gas pressure directlyupstream of an exit (the distal end 135) of the measurement nozzle 110and directly downstream of the exit of the measurement nozzle 110 in anannulus area 155 substantially equal to or greater than a thresholdvalue corresponding to a maximum expected gas pressure directlydownstream of the exit of the measurement nozzle 110.

The gas gauge 100 may further comprise a sealed housing 160 to enclosethe measurement gas flow channel 105 and the pressure sensor 127 forproviding a contamination barrier in the vacuum environment. The gasgauge 100 in the vacuum environment may operate choked with a desiredflow of the gas at the distal end 135 of the measurement nozzle 110 suchthat the gas supply 120 provides a constant flow of air or Nitrogen. Thesecondary pressure port 122 may be configured to vent gas from the gassupply 120 in the vacuum environment. When the target surface 140 is ofa wafer, the gas gauge 100 may enable establishing of a best plane foran exposure step in a wafer processing procedure being performed by alithography apparatus.

FIG. 2 schematically illustrates a side view of a gas gauge such as anair gauge 200 for use in a vacuum environment according to oneembodiment of the present invention. The air gauge 200 may include asecondary gas flow channel 205. The secondary gas flow channel 205 maycomprise a secondary nozzle 210 in the secondary gas flow channel 205.The secondary nozzle 210 may be operatively coupled to the pressuresensor 127 to enable the pressure sensor 127 to measure the differentialpressure of the volumetric flow between a first pressure level of themeasurement nozzle 110 and a second pressure level of the secondarynozzle 210.

In the air gauge 200, the secondary nozzle 210 may either limit a rangeof measurement for the differential pressure that the pressure sensor127 to obtain or counteract a variation in a constant mass flow to themeasurement nozzle 110 from the gas supply 120. The air gauge 200 mayprovide a measure of an offset distance in the gap 130 based on a signalof the differential pressure from the pressure sensor 127 fordetermining a focus position by mapping topography of a wafer.

Consistent with one embodiment of the present invention, tire secondarygas flow channel 205 of the air gauge 200 further comprises a secondflow restrictor 125 a located upstream to the pressure sensor 127. Thesecond flow restrictor 125 a may be configured to depress pressure at aninlet 150 a of the secondary gas flow channel 205 for gas flowing at asubstantially constant flow rate from the gas supply 120. The inlet 150a of the secondary gas flow channel 205 may be configured to receive thegas from the gas supply 120. The secondary nozzle 210 may be configuredto vent gas in the vacuum environment from the gas supply 120.

FIG. 3 schematically illustrates a plot 300 showing a flow rate of air(Flow) as a function of the pressure (Pr) to realize a sonically chokedflow condition 305 of a volumetric flow according to one embodiment ofthe present invention. The sonically choked flow condition 305 of avolumetric air flow from the gas supply 120 that may be supplying airbeing choked by the first flow restrictor 125 is a dynamic conditioncaused by the Venturi effect. When a flowing gas at a certain pressureand temperature flows through the first flow restrictor 125 (such as ahole in an orifice plate or a valve in a pipe) into a lower pressureenvironment, under the conservation of mass the flow velocity mustincrease for initially subsonic upstream conditions as it flows throughthe smaller cross-sectional area of the first flow restrictor 125. Atthe same time, the Venturi effect causes the pressure to decrease.

The sonically choked flow condition 305 is a limiting condition whichoccurs when the mass flux will not increase with a further decrease inthe downstream pressure environment. For homogenous gases such as air orNitrogen, the physical point at which the choking occurs for adiabaticconditions is when the exit plane velocity is at sonic conditions or atleast at a Mach number of 1. Such choked flow of gases is useful in manyengineering applications because the mass flow rate is independent ofthe downstream pressure, depending only on the temperature and pressureon the upstream side of the first flow restrictor 125. Under thesonically choked flow condition 305, a calibrated first flow restrictor125 may be used to produce a particular mass flow rate.

In accordance with one embodiment of the present invention, a sensorbased on the air gauge 200 may measure “true” offset distance as the gap130 between the distal end 135 of the measurement nozzle 110 and a closeproximity surface, such as the target surface 140. The air gauge 200 ina vacuum environment may operate choked. For example, a EUV vacuumenvironment may be defined by a max flow =0.1 lpm with “ambient”pressure <0.1 mbar in one specific application for lithographicprocessing. The choked flow may be defined as Mach 1 at the exit or thedistal end 135. A choke boundary may be expected at the measurementnozzle 110.

For creating the sonically choked flow condition 305, a ratio betweenupstream pressure (P_(upstream)) and downstream pressure(P_(downstream)) may be maintained asP_(upstream)/P_(downstream) >1.893. Other governing equations for thesonically choked flow condition 305 include: Restrictor (ΔP=k Q L/A);Friction (ΔP=½ fL/D ρ(Q/A)2); Area change (ΔP=½ ρ(Q/A)2(A2/A1)2−1);Nozzle (ΔP=Pin(1−(1+(γ−1)/2 M2))(γ/γ+1)). Likewise, the operationalparameters may include Pressure: 100 mbar to 0.1 mbar, ˜100 mbar netdrop; Flow: 0.1 nlpm, the Gap 130: 135 μm gap; Pneumatic gain:(pressure) 126 mPa/nm (flow) 4.5×10−7 nlpm/nm (may not be measurable asflow). In this way, the air gauge 200 blowing into vacuum becomesfeasible by using the pressure sensor 127 instead of a mass flow sensor.

FIG. 4 schematically illustrates formation of an annulus 155 a accordingto an embodiment of the present invention. The annulus 155 a refers to aring-shaped geometric figure, or more generally, a ring-shaped object.The annulus 155 a may be a void between the measurement nozzle 110 andthe target surface 140 immediately surrounding it where a gas such asair or Nitrogen can flow from the gas supply 120 after it is beingchoked.

In operation, the gas supply 120 and the first flow restrictor 125 maybe configured to maintain a ratio between gas pressure directly upstream(P_(upstream)) of an exit (the distal end 135) of the measurement nozzle110 and directly downstream (P_(downstream)) of the exit of themeasurement nozzle 110 in the annulus's 155 a area. This ratio may bemaintained substantially equal to or greater than a threshold valuecorresponding to a maximum expected gas pressure directly downstream ofthe exit of the measurement nozzle 110.

In the air gauge 200, the measurement nozzle 110 may be set at a smalloperating gap 130 and the inlet 150 of the measurement nozzle 110 may bethrottled using a porous restrictor such as the first flow restrictor125 set to produce a fixed nozzle inlet pressure at a desired flow rate.The measurement nozzle 110 may be operated so as to reach sonicconditions at a nozzle exit (the distal end 135), which develops aunique relation between a Flow 400, a Pressure as shown by an arrow 405and an annulus area 410. By holding the flow rate constant, the Pressure405 may be solely a function of the annulus area. 410 which isindicative of a height or the gap 130 that the measurement nozzle 110 isoperating at.

Accordingly, the air gauge 200 into vacuum may operate in a choked flowregime, producing sufficient pneumatic gain to function while using thepressure sensor 127. Consistent with one embodiment of the presentinvention, the air gauge 200 may be configured to produce 3-4 times morepneumatic gain than a conventional air gauge sensor.

FIG. 5 schematically illustrates a plot 500 showing the Pressure 405 asa function of the gap 130 for indicating gain according to oneembodiment of the present invention, in one embodiment of the presentinvention, pneumatic data for operation in atmospheric conditions mayinclude the Flow 400=58 Slpm, the Pressure 405=312 kPa, and the Gain=250 pa/um.

FIG. 6 schematically shows a block diagram of a method 600 fordetermining a focus position to set focus in a photolithographicexposure tool by mapping topography of a wafer in a vacuum environmentaccording to one embodiment of the present invention. The steps of themethod 600 are for illustrative purpose only, and do not have to takeplace in the order shown. There may be additional intermediate stepsthat are not shown in FIG. 6. Some of the steps may be optional, and/orspecific to particular embodiments. All embodiments may not use all thesteps shown in FIG. 6. Additionally, components shown in FIGS. 1 and 2are configured to execute various functional steps shown in FIG. 6.However, the method 600 is not limited to the embodiment shown in FIGS.1 and 2, and can be executed by other system embodiments as well.

As shown in block 605, the method 600 may comprise providing a supply ofgas such as air or Nitrogen from the gas supply 120.

As shown in block 610, the method 600 may further comprise using in thevacuum environment the gas gauge 100 having the measurement nozzle 110.The measurement nozzle 110 may be configured to operate at a sonicallychoked flow condition of a constant volumetric flow being supplied bythe gas supply 120. The gas gauge 100 may include the pressure sensor127, which may be operatively coupled to the measurement nozzle 110downstream from the sonically choked flow condition of the volumetricflow to measure a differential pressure of the volumetric flow. Themeasure of the differential pressure may indicate height of the gap 130between the distal end 135 of the measurement nozzle 110 and the target145's surface 140 such as a substrate surface proximal to themeasurement nozzle 110. As shown in block 615, the method 600 mayadditionally comprise exposing the substrate to a beam of radiationcomprising a pattern in its cross section.

As shown in block 620, exposing the substrate to a beam of radiationcomprising a pattern in its cross section may further comprisepositioning the measurement nozzle and the substrate such that the gasvented from the measurement nozzle 110 impinges on the substrate. Asshown in block 625, the method 600 may also comprise measuring one ormore aspects of the substrate by using the gas gauge 100 prior toexposing the substrate.

FIG. 7 schematically depicts a lithographic apparatus 700 for using theair gauge 200 shown in FIG. 2 in a vacuum environment according to oneembodiment of the present invention. The lithography apparatus 700 maycomprise the gas supply 120 shown in FIG. 1, the gas gauge 100 as shownin FIG. 1 for use in a vacuum environment having the measurement nozzle110 configured to operate at a sonically choked flow condition of aconstant volumetric flow being supplied by the gas supply 120. Asdescribed above, the gas gauge 100 may include the pressure sensor 127operatively coupled to the measurement nozzle 110 downstream from thesonically choked flow condition of the volumetric flow to measure adifferential pressure of the volumetric flow. This measurement mayprovide an indication of the gap 130 between the distal end 135 of themeasurement nozzle 110 and a substrate such as the target 145 proximalthereto.

Within the lithography apparatus 700, a conventional exposure stationmay be configured to expose the substrate to a beam of radiationcomprising a pattern in its cross section. Additionally the lithographyapparatus 700 may comprise a conventional measurement station which maybe configured to measure one or more aspects of the substrate by usingthe gas gauge prior to exposing the substrate. The measurement nozzle110 and the substrate may be positioned such that the gas vented fromthe measurement nozzle 110 impinges on the substrate such as the target145. The lithography apparatus 700 may further comprise a conventionalsubstrate stage compartment that is configured to comprise a substratestage configured to support the substrate during illumination of thesubstrate with a beam with a patterned cross section.

In one embodiment, the lithography apparatus 700 may further comprise aconventional lithography leveling system that includes the gas gauge100. In another embodiment, the lithography apparatus 700 may furthercomprise a conventional lithography topography mapping device thatincludes the gas gauge 100. In an alternate embodiment, the lithographyapparatus 700 may further comprise a conventional proximity sensor forlithography that includes the gas gauge 100.

Consistent with one embodiment of the present invention, the apparatus700 may comprise an illumination system (illuminator) IL configured tocondition a radiation beam B (e.g. UV radiation or EUV radiation); asupport structure (e.g. a mask table) MT constructed to support apatterning device (e.g. a mask) MA and connected to a first positionerPM configured to accurately position the patterning device in accordancewith certain parameters; a substrate table (e.g. a wafer table) WTconstructed to hold a substrate (e.g. a resist-coated wafer) W andconnected to a second positioner PW configured to accurately positionthe substrate in accordance with certain parameters; and a projectionsystem (e.g. a refractive projection lens system) PS configured toproject a pattern imparted to the radiation beam B by patterning deviceMA onto a target portion C (e.g. comprising one or more dies) of thesubstrate W.

The illumination system may include various types of optical components,such as refractive, reflective, magnetic, electromagnetic, electrostaticor other types of optical components, or any combination thereof, fordirecting, shaping, or controlling radiation.

The support structure supports, i.e. bears the weight of, the patterningdevice. It holds the patterning device in a manner that depends on theorientation of the patterning device, the design of the lithographicapparatus 700, and other conditions, such as for example whether or notthe patterning device is held in a vacuum environment. The supportstructure can use mechanical, vacuum, electrostatic or other clampingtechniques to hold the patterning device. The support structure may be aframe or a table, for example, which may be fixed or movable asrequired. The support structure may ensure that the patterning device isat a desired position, for example with respect to the projectionsystem. Any use of the terms “reticle” or “mask” herein may beconsidered synonymous with the more general term “patterning device.”

The term “patterning device” used herein should be broadly interpretedas referring to any device that can be used to impart a radiation beamwith a pattern in its cross-section such as to create a pattern in atarget portion of the substrate. It should be noted that the patternimparted to the radiation beam may not exactly correspond to the desiredpattern in the target portion of the substrate, for example if thepattern includes phase-shifting features or so called assist features.Generally, the pattern imparted to the radiation beam will correspond toa particular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. Examples ofpatterning devices include masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions. The tilted mirrorsimpart a pattern in a radiation beam which is reflected by the mirrormatrix.

The term “projection system” used herein should be broadly interpretedas encompassing any type of projection system, including refractive,reflective, catadioptric, magnetic, electromagnetic and electrostaticoptical systems, or any combination thereof, as appropriate for theexposure radiation being used, or for other factors such as the use ofan immersion liquid or the use of a vacuum. Any use of the term“projection lens” herein may be considered as synonymous with the moregeneral term “projection system.”

As here depicted, the apparatus 700 is of a reflective type (e.g.employing a reflective mask). Alternatively, the apparatus 700 may be ofa transmissive type (e.g. employing a transmissive mask).

The lithographic apparatus 700 may be of a type having two (dual stage)or more substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus 700 may also be of a type wherein at least aportion of e substrate may be covered by a liquid having a relativelyhigh refractive index, e.g. water, so as to fill a space between theprojection system and the substrate. An immersion liquid may also beapplied to other spaces in the lithographic apparatus 700, for example,between the mask and the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems. The term “immersion” as used herein does not meanthat a structure, such as a substrate, must be submerged in liquid, butrather only means that liquid is located between the projection systemand the substrate during exposure.

Referring to FIG. 7, the illuminator IL receives a radiation beam from aradiation source SO. The source and the lithographic apparatus 700 maybe separate entities, for example when the source is an excimer laser.In such cases, the source is not considered to form part of thelithographic apparatus and the radiation beam is passed from the sourceSO to the illuminator IL with the aid of a beam delivery system BDcomprising, for example, suitable directing mirrors and/or a beamexpander. In other cases the source may be an integral part of thelithographic apparatus 700, for example when the source is a mercurylamp. The source SO and the illuminator IL, together with the beamdelivery system BD if required, may be referred to as a radiationsystem.

The illuminator IL may comprise an adjuster AD for adjusting the angularintensity distribution of the radiation beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. In addition, the illuminator IL maycomprise various other components, such as an integrator IN and acondenser CO. The illuminator may be used to condition the radiationbeam, to have a desired uniformity and intensity distribution in itscross-section.

The radiation beam B is incident on the patterning device (e.g., maskMA), which is held on the support structure (e.g., mask table MT), andis patterned by the patterning device. Having traversed the mask MA, theradiation beam B passes through the projection system PS, which focusesthe beam onto a target portion C of the substrate W. With the aid of thesecond positioner PW and position sensor IF2 (e.g. an interferometricdevice, linear encoder or capacitive sensor), the substrate table WT canbe moved accurately, e.g. so as to position different target portions Cin the path of the radiation beam B. Similarly, the first positioner PMand another position sensor IF1 can be used to accurately position themask MA with respect to the path of the radiation beam B, e.g. aftermechanical retrieval from a mask library, or during a scan. In general,movement of the mask table MT may be realized with the aid of along-stroke module (coarse positioning) and a short-stroke module (finepositioning), which form part of the first positioner PM. Similarly,movement of the substrate table WT may be realized using a long-strokemodule and a short-stroke module, which form part of the secondpositioner PW. In the case of a stepper (as opposed to a scanner) themask table MT may be connected to a short-stroke actuator only, or maybe fixed. Mask MA and substrate W may be aligned using mask alignmentmarks M1, M2 and substrate alignment marks P1, P2. Although thesubstrate alignment marks as illustrated occupy dedicated targetportions, they may be located in spaces between target portions (theseare known as scribe-lane alignment marks). Similarly, in situations inwhich more than one die is provided on the mask MA, the mask alignmentmarks may be located between the dies.

The depicted apparatus 700 could be used in at least one of thefollowing modes:

1. In step mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theradiation beam is projected onto a target portion C at one time (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

2. In scan mode, the mask table MT and the substrate table WT arescanned synchronously while a pattern imparted to the radiation beam isprojected onto a target portion C a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT may be determined by the (de-)magnification and image reversalcharacteristics of the projection system PS. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

3. In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the radiation beam isprojected onto a target portion C. In this mode, generally a pulsedradiation source is employed and the programmable patterning device isupdated as required after each movement of the substrate table WT or inbetween successive radiation pulses during a scan. This mode ofoperation can be readily applied to maskless lithography that utilizesprogrammable patterning device, such as a programmable mirror array of atype as referred to above. Combinations and/or variations on the abovedescribed modes of use or entirely different modes of use may also beemployed.

The above description refers to light, light sources and beams of light.It will be appreciated that the light referred to is not limited tolight having a visible wavelength, and can include other wavelengthsincluding ultraviolet light or infrared light which are suitable forlithography, as discussed above.

For example, an EUV photolithographic system may include an EUV source.The EUV photolithographic system may also includes image opticsincluding mirrors, a pupil, a reflective reticle mounted on a reticlestage (RS) with an image of a pattern to be imaged onto a wafer 805, andprojection optics (PO) mirrors. The EUV radiation is then projected ontoa wafer, which is mounted on a wafer stage (WS, not shown). It will beappreciated that the reticle is reflective in EUV systems, unlikephotolithographic systems operating at longer wavelengths, such as deepultraviolet, or visible, where the reticle is usually transmissive,although the invention is applicable to both types of reticles.

Although specific reference can be made in this text to the use oflithographic apparatus 700 in the manufacture of a specific device(e.g., an integrated circuit or a flat panel display), it should beunderstood that the lithographic apparatus described herein can haveother applications. Applications include, but are not limited to, themanufacture of integrated circuits, integrated optical systems, guidanceand detection patterns for magnetic domain memories, flat-paneldisplays, liquid-crystal displays (LCDs), thin-film magnetic heads,micro-electromechanical devices (MEMS), etc. Also, for instance in aflat panel display, the present apparatus can be used to assist in thecreation of a variety of layers, e.g. a thin film transistor layerand/or a color filter layer.

The skilled artisan will appreciate that, in the context of suchalternative applications, any use of the terms “wafer” or “die” hereinmay be considered as synonymous with the more general terms “substrate”or “target portion”, respectively. The substrate referred to herein maybe processed, before or after exposure, in for example a track (a toolthat typically applies a layer of resist to a substrate and develops theexposed resist), a meteorology tool and/or an inspection tool. Whereapplicable, the disclosure herein may be applied to such and othersubstrate processing tools. Further, the substrate may be processed morethan once, for example in order to create a multi-layer IC, so that theterm substrate used herein may also refer to a substrate that alreadycontains multiple processed layers.

Although specific reference may have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention may be used in otherapplications, for example imprint lithography, and where the contextallows, is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device may be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

Although specific reference can have been made above to the use ofembodiments of the invention in the context of optical lithography, itwill be appreciated that the invention can be used in otherapplications, for example imprint lithography, where the context allows,and is not limited to optical lithography. In imprint lithography atopography in a patterning device defines the pattern created on asubstrate. The topography of the patterning device can be pressed into alayer of resist supplied to the substrate whereupon the resist is curedby applying electromagnetic radiation, heat, pressure or a combinationthereof. The patterning device is moved out of the resist leaving apattern in it after the resist is cured.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of or about 365, 355, 248, 193, 157 or 126 nm) andextreme ultra-violet (EUV) radiation (e.g. having a wavelength in therange of 5-20 nm), as well as particle beams, such as ion beams orelectron beams.

The term “lens”, where the context allows, may refer to any one orcombination of various types of optical components, includingrefractive, reflective, magnetic, electromagnetic and electrostaticoptical components.

While specific embodiments of the invention have been described above,it will be appreciated that the invention can be practiced otherwisethan as described. For example, the invention can take the form of acomputer program containing one or more sequences of machine-readableinstructions describing a method as disclosed above, or a data storagemedium (e.g. semiconductor memory, magnetic or optical disk) having sucha computer program store therein.

CONCLUSION

While various embodiments of the present invention have been describedabove, it should be understood that they have been presented by way ofexample only, and not limitation. It will be apparent to persons skilledin the relevant art that various changes in form and detail can be madetherein without departing from the spirit and scope of the invention.Thus, the breadth and scope of the present invention should not belimited by any of the above-described exemplary embodiments, but shouldbe defined only in accordance with the following claims and theirequivalents.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections can set forth one or more,but not all exemplary embodiments of the present invention ascontemplated by the inventor(s), and thus, are not intended to limit thepresent invention and the appended claims in any way.

What is claimed is:
 1. A gas gauge configured to be used in a vacuumenvironment, comprising: a measurement nozzle in a measurement gas flowchannel, the measurement nozzle being configured to provide a gas flowsupplied by a gas supply; a pressure port; and a pressure sensoroperatively coupled between the pressure port and the measurement nozzleconfigured to measure a differential pressure between a first pressurewithin the measurement nozzle and a second vacuum pressure at thepressure port to provide an indication of a gap between a distal end ofthe measurement nozzle and a target surface proximal thereto.
 2. The gasgauge of claim 1, further comprising: a secondary gas flow channelcomprising a secondary nozzle, the secondary nozzle operatively coupledto the pressure sensor to allow the pressure sensor to measure thedifferential pressure of the gas between a first pressure level of themeasurement nozzle and a second pressure level of the secondary nozzle.3. The gas gauge of claim 2, wherein the secondary nozzle is configuredto perform at least one of: limiting a range of measurement for thedifferential pressure that the pressure sensor obtains, andcounteracting a variation in a constant mass flow to the measurementnozzle from the gas supply.
 4. The gas gauge of claim 2, wherein thesecondary gas flow channel of the gas gauge further comprises: arestrictor located upstream of the pressure sensor, the restrictor beingconfigured to depress pressure at an inlet of the secondary gas flowchannel for gas flowing at a substantially constant flow rate from thegas supply, the inlet of the secondary gas flow channel being configuredto receive the gas from the gas supply.
 5. The gas gauge of claim 4,wherein the gas gauge is choked with a desired flow of the gas at thedistal end of the measurement nozzle such that the gas supply provides aconstant flow of air or nitrogen, the secondary nozzle being configuredto vent gas in the vacuum environment from the gas supply.
 6. The gasgauge of claim 1, wherein a measure of an offset distance of the gap isproduced based on a signal associated with the differential pressuremeasured from the pressure sensor.
 7. The gas gauge of claim 1, whereinthe measurement nozzle is further configured to operate at a sonicallychoked flow condition.
 8. The gas gauge of claim 7, wherein themeasurement gas flow channel of the gas gauge further comprises: arestrictor located upstream of the pressure sensor and configured todepress pressure at an inlet of the measurement gas flow channel whileallowing the measurement nozzle to reach the sonically choked flowcondition for gas flowing at a substantially constant flow rate from thegas supply, the inlet of the measurement gas flow channel beingconfigured to receive the gas from the gas supply.
 9. The gas gauge ofclaim 8, wherein the gas supply and the restrictor are configured tomaintain a ratio between gas pressure directly upstream of an exit ofthe measurement nozzle and directly downstream of the exit of themeasurement nozzle in an annulus area substantially equal to or greaterthan a threshold value corresponding to a maximum expected gas pressuredirectly downstream of the exit of the measurement nozzle.
 10. The gasgauge of claim 9, wherein the ratio between gas pressure directlyupstream of an exit of the measurement nozzle and directly downstream ofthe exit of the measurement nozzle in an annulus area is greater than1,893.
 11. The gas gauge of claim 1, further comprising: a sealedhousing configured to enclose the measurement gas flow channel and thepressure sensor and to provide a contamination barrier in the vacuumenvironment.
 12. A method comprising: using, in a vacuum environment, agas gauge having a measurement nozzle configured to provide a gas flowsupplied by a gas supply; measuring, using a pressure sensor, adifferential pressure between a first pressure within the measurementnozzle and a second vacuum pressure at a pressure port, the pressuresensor being coupled between e pressure port and the measurement nozzle;and producing, using the pressure sensor, an indication of a gap betweena distal end of the measurement nozzle and a target surface proximalthereto.
 13. The method of claim 12, further comprising: impinging gasvented from the measurement nozzle on the target surface.
 14. The methodof claim 12, further comprising: measuring an offset distance of the gapbased on a signal associated with the differential pressure measuredfrom the pressure sensor.
 15. The method of claim 14, furthercomprising: determining one or more aspects of the target surface fromthe measured offset distance of the gap.
 16. The method of claim 12,wherein using a gas gauge having a measurement nozzle comprises using agas gauge having a measurement nozzle configured to operate at asonically choked flow condition.